Checkbox in Command (c-shaped)

When you select Command (c-shaped) from the parameter dialog while creating a custom block, there is a checkbox next to "Single Input." What does this checkbox do? Not sure if I missed anything in the Snap! Manual.

If it looks like an arrow, that puts a little "loop indicator" at the bottom of the block. Look at the if statements and then look at the for statements. You can see that arrow.

It doesn't do anything, it just puts a picture of an arrow at the bottom of the block.

Can you help me creating c-shaped blocks?

On the block creation menu, click one of the + signs to add an input. Give your input a name, then open the full menu. Select "Command (c-shape)" and click OK. Then, in the definition, you can run the code inside like this:
run (code)::control